Piezoelectric or electret transducers are well known as such. They are typically used to convert a mechanical stimulus (such as vibration, movement, strain etc.) into an electrical signal.
US patent application 2010/0295563 A1 discloses an occupant detection system with an electrode arrangement for a seat of an automotive vehicle, wherein the electrode arrangement includes a first electrode, a second electrode and an electric insulator layer sandwiched between the first and second electrodes. When the electrode arrangement is in place in the seat, the first electrode forms with vehicle ground a first capacitor having a first capacitance influenceable by an occupying item in the detection region through interaction of the occupying item with the electric field, the first electrode forms with the second electrode a second capacitor having a second capacitance and the second electrode forms a first capacitor plate of a third capacitor having a third capacitance. The second plate of the third capacitor may be formed by conductive surfaces at vehicle ground potential or a third electrode formed by further conductive surface in the vehicle seat, e.g. a seat heater, the seat pan or a third electrode arranged behind the second electrode (as seen from the first electrode). The fluctuations of at least one of the first, second and third capacitances are measured and the frequency spectrum of the measured fluctuations is analyzed in order to obtain an indicator of the seat occupancy state. According to a preferred embodiment of the invention of US 2010/0295563 A1, the electric insulator layer between the first and the second electrode comprises an electret layer and/or a piezoelectric layer. In this configuration, the evaluation circuit determines, among others, a current induced in the first and/or second electrode by the electret or piezoelectric layer (in response to pressure being applied to the electret or piezoelectric layer) and derives the fluctuations of the second capacitance from the induced current.
U.S. Pat. No. 6,531,884 B1 relates to a diagnostics device for testing a piezoelectric sensor. The diagnostics device includes an AC source that applies an AC signal to the piezoelectric sensor at two or more different frequencies. Measurement circuitry coupled to the piezoelectric sensor measures a response of the sensor to the applied AC signal and provides a measured output related to a sensor resistance and a sensor capacitance of the piezoelectric sensor. Diagnostic circuitry provides a diagnostic output as a function of the measured output. As a disadvantage of the disclosed system, the measurement mode and the diagnostics mode must be carried out one after the other.